AdaptiSPECT: a Preclinical Imaging System

Abstract

This dissertation addresses the design, development, calibration and performance evaluation of a pre-clinical imaging system called AdaptiSPECT. Single-Photon Emission Computed Tomography (SPECT) systems are powerful tools for multiple applications in small-animal research, ranging from drug discovery to fundamental biology. Traditionally, pinhole SPECT systems are designed with fixed imaging characteristics in terms of sensitivity, resolution and size of the field of view, that are dictated by the hardware configuration of the system. The SPECT system described in this dissertation can change its hardware configuration in response to the subject data it is acquiring in order to improve the imaging performance. We employed 16 modular gamma-ray detectors, each of which consists of a NaI:Tl scintillation crystal, a fused silica lightguide, and an array of 9 PMTs. The camera is designed to work with maximum-likelihood position estimation methods. These detectors are arranged into 2 rings of 8 detectors around an adjustable pinhole aperture. The aperture itself comprises three cylinders of different diameters, each with pinholes of different diameters. The three aperture cylinders are stacked together along the imager axis, and selection of the appropriate ring of pinholes is carried out by translating the entire aperture assembly. In addition, some sections of the aperture are fitted with shutters to open or close additional pinholes that increase sensitivity. We reviewed the method used to calibrate AdaptiSPECT, and proposed a new interpolation scheme specific to adaptive SPECT imaging systems where the detectors can move to multiple locations, that yields system matrices for any configuration employed during adaptive imaging. We evaluated the performances of AdaptiSPECT for various configurations. The magnification of the system ranges from 1.2 to 11.1. The corresponding resolution ranges from 3.2 mm to 0.6 mm, and the corresponding transaxial field-of-view ranges from 84 mm to 10 mm. The sensitivity of the system varies from 220 cps/MBq to 340 cps/MBq for various configurations. Imaging of a mouse injected with a bone radiotracer revealed the finer structures that can be acquired at higher magnifications, and illustrated the ability to conveniently image with a variety of magnifications during the same study. In summary, we have brought the concept of an adaptive SPECT imaging system as it was originally described by Barrett et al. in 2008 to life. We have engineered a system that can switch configurations with speed, precision, and repeatability suitable to carry out adaptive imaging studies on small animals, thus opening the door to a new research and medical imaging paradigm in which the imager hardware is adjusted on the fly to maximize task-performance for a specific patient, not, as currently, an ensemble of patients.